This invention concerns a method for ascertaining the position of a mobile closing element of an injection valve in a motor vehicle engine.
Storage injection systems are used for supplying fuel for internal combustion motors which operate at very high injection pressures. Such injection systems are known, for example, as common rail systems. These injection systems are distinguished in that the fuel is conveyed with a high pressure pump into a pressure storage unit jointly allocated to all cylinders of the motor, from which the injection valves on the individual cylinders are supplied. The injection valves are frequently also called injectors. The opening and closing of the injection valves are usually electrically controlled, for example with the aid of piezoelectric elements as actuators.
A control valve can be connected between the nozzle element, with the nozzle needle which opens and closes the injection apertures in the injection valve, and the piezoelectric actuator, as a closing element in injection valves or injectors. The control valve serves to bring about the opening and closing of the actual fuel injection valve hydraulically. That in particular means establishing the beginning and the end of the injection process exactly in time. The injection valve should, for example, open and, at the end of the injection process, rapidly close in a controlled manner. The injection of minute amounts of fuel for preliminary injection before the actual injection, with which the combustion process can be optimized, should also be possible. The closing element can nonetheless also be arranged in another form and at another place on the injection valve, for example as a valve hinged cover or needle valve at the valve exit. An injector needle can be used as a closing element. The injection valve can be constructed as a needle valve.
A method for regulating a fuel injection process with a fuel injection valve for a motor vehicle internal combustion engine is known from German Patent 199 30 309 C2. A control valve as closing element is activated by a piezoelectric element as an actuator for opening the injection valve. The piezoelectric element is electrically actuated to change the state of the closing element. Following this actuation, the voltage on the piezoelectric element is measured and the beginning of injection or the needle opening of the injection valve is ascertained on the basis of the voltage measured.
An object of the invention is to make it possible to simply, exactly, and rapidly ascertain the position of a closing element of an injection valve in a motor vehicle engine.
This object is accomplished, in a method in which a closing element is driven using a piezoelectric element for opening or closing an injection valve, by determining a voltage signal allocated to an electric voltage on the piezoelectric element, and determining the position of the closing element from the voltage signal ascertained as a change of the voltage signal, based upon a change in the electric voltage on the piezoelectric element, is brought about by a two-way valve arranged on the injection valve as a function of the position of the closing element.
Preferably, the method for ascertaining the position of a closing element in connection with a fuel injection valve is conducted with a motor vehicle internal combustion engine. The closing element is driven using a piezoelectric element to open and close the injection valve. With the closing element, preferably an injector needle is guided one dimensionally in a mounting in the longitudinal direction of the needle. Nonetheless, the method is not restricted to the special case of an injector needle, and can also be conducted with other closing elements such as, for example, controllable hinged covers or ball valves.
The piezoelectric element is electrically actuated by means of a control apparatus for opening and closing the injection valve. An electric voltage is recorded on the piezoelectric element, and an output signal is allocated to the electrical voltage recorded. The position of the closing element is ascertained from a voltage signal representing the voltage incident upon the piezoelectric element. Ascertaining the position of the closing element can take place simply and exactly, since a change of the electric voltage recorded on the piezoelectric element, and consequently a change in the voltage signal, is brought about using a two-way valve arranged on the injection valve as a function of the position of the closing element.
A further advantage is that an additional sensor element and an evaluation of additional sensor signals can be dispensed with.
According to one feature of the invention, the alteration of the electrical voltage is brought about on the piezoelectric element using a change in pressure in liquid surrounding the piezoelectric element, caused by an opening or closing of the two-way valve. As an alternative to this, the pressure change can take place in a liquid standing in contact with a transmission medium, whereby the transmission medium stands in mechanical contact with the piezoelectric element.
It is especially advantageous if the pressure change takes place abruptly during opening or closing of the two-way valve. When the pressure in the liquid surrounding the piezoelectric element changes very rapidly, the voltage signal allocated to the voltage on the piezoelectric element has a pulse-like course, that is, the voltage signal changes rapidly at points in time allocated to the pressure change. Thus, it has a great “elevation” or slope. A voltage signal with a pulse-like course makes an especially exact and reliable determination of the position of the closing element possible.
It is especially advantageous if the temporal derivation of the voltage signal is adduced for ascertaining the position of the closing element. Moreover, it is possible to ascertain the position of the closing element rapidly and exactly in a pulse-like course of the voltage signal.
In one embodiment, a first portion of the batch of fuel from the injection valve, standing under pressure and to be injected into a useful space, is guided through a surrounding space formed around the piezoelectric element. A second part of the batch of fuel to be injected is led into a bypass line.
According to a further feature, the piezoelectric element is driven using a current drive for controlling the closing device of the injection valve. The closing device of the injection valve is controlled by specifying the current flowing through the piezoelectric element. The electric voltage in reference to the piezoelectric element is recorded and evaluated for ascertaining the position of the closing element. As an alternative to recording the piezoelectric voltage while administering current, recording the piezoelectric voltage can also take place in pauses in the administration of current. For this purpose, the piezoelectric element can be electrically separated from the current supply in pauses in administration of current so that recording of the piezoelectric voltage is possible on the electrically free piezoelectric element.
Preferably ascertaining the position of the closing element will be adduced for regulating the course of injection of an injection value. Regulation of the course of injection can take place to reduce fuel consumption, to diminish toxic emissions or, for example, to optimize, and especially reduce, the motor noise.
Any desired closing element, such as a valve, can be used for the closing element of the injection valve but a longitudinally displaceable injector needle is preferably used.
In a further configuration, the two-way valve includes a recess in the injector needle which, in a first position of the injector needle, interacts with a recess in the needle guide such that a liquid can flow from the one recess into the other. In a second position of the injector needle, flowing of a liquid from one recess into the other recess is prevented.
An especially advantageous application of the method arises in measuring the needle position of an injector needle driven using a piezoelectric element in an injector. With regard to their dynamic behavior, piezoelectric actuators make possible high positioning forces and short response times in tightly restricted injector contours, such as preliminary injection and postinjection, to reduce the development of noise and toxic substances during the course of combustion. Here the exact knowledge of the position of the injector needle in relation to the camshaft adjustment is especially advantageous for injection periods smaller than 100 μs.
A force F(t) acting upon the piezoelectric element, preferably in an axial direction, is transformed into piezoelectric voltage. This possesses the advantage that an additional installation of a piezoelectric element as a sensor can be dispensed with.
With the aid of the equationsΔ1=s33E·1/A·F+d33·n·up  (1)ip=d33·n·dF/dt+ε33T·n2·A/1·dup/dt  (2)in which Δ1 is a change in length of the piezoelectric element, Dn is an electric displacement flux density on the piezoelectric element, 1 the total length of the piezoelement, Em the electrical field strength on the piezoelectric element, A the surface of the piezoelectric element, Si the mechanical extension, F the external force on surface A and Tj the voltage tensor, up the piezoelectric voltage, qp=∫ip dt the electric charge, din the piezoelectric coefficient, ip the piezoelectric current, and CE=ε33T·n2·A/1 the replacement capacitance of the piezoelectric element.                                    Furthermore the following applies for the dielectric constants:εmmT=∂Dn/∂Em|Twith the number of ceramic layers n and for the elasticity coefficient:sijE=∂Si/∂Tj|E.                        
The description of the connections between the change in length Δ1 of the piezoelectric stack, the force F upon the piezoelectric stack, the piezoelectric current ip and the piezoelectric voltage up takes place through elimination of the change in force over time dF/dt, to detect the influence of the needle motion in the direction of its speed of motion dΔ1/dt on the temporal change of the piezoelectric voltage duP/dt.duP/dt=iP·1/A·1/(ε33T·n2)−dF/dt·1/A·d33/(ε33T·n)  (3)dF/dt=dΔ1/dt·A/(1·s33E)−dup/dt·A·d33·n/(1·s33E)  (4)The equation is solved according to dup/dt. The leading magnitudes are then iP and dΔ1/dt. The equation for the temporal change of the piezoelectric voltage duP/dt as a function of piezoelectric current iP and the change in length per unit of time dΔ1/dt then reads:duP/dt={iP·1/(A·n)−dΔ1/dt·d33/s33E}·{1/n·s33E/(s33E·ε33T−(d33)2)}  (5)By ascertaining the change in voltage on the piezoelectric element when evaluating this equation, an especially efficient ascertaining of the needle position of the injection valve is possible. For this, a specifiable disturbance is introduced into an existing piezoelectric injector system such that the originally unimpeded flow of fuel, proceeding from the common rail to the motor cylinder, assumes a pulse-like course through the two-way valve. This pressure pulse resulting as a consequence of force pulses in the liquid surrounding the piezoelectric injector acts as a change in the ambient pressure upon the piezoelectric element and is imaged in the voltage signal u(t) in the electric feeder lines of the piezoelectric element as a measured magnitude. A temporal derivation of the piezoelectric voltage makes possible a good localization of the state of the needle, especially for the beginning and end of an injection. The measuring signal stands out clearly in relation to interference signals, and an especially high resolution of the position is attainable.